Generally, in a working machines which is provided with an operator's cab, for example, as in the case of hydraulic power shovels or the like, a box-like body of an operator's cab is mounted on a support frame of a working machine body, and a cushioning support means is located between the box-like cab body and the support frame to damp transmission of vibrations from the support frame to the operator's cab. By suppressing transmission of vibrations to the cab body, including vibrational excavation reaction forces which occur during ground excavating operations and vibrations which occur while the machine is in travel on the ground surface, the cushioning support means of this sort contribute to prevent degradations in amenity of the operating space within the cab and in operationability on the control levers which are provided in the operator's cab.
As for conventional working machines of the sort which are equipped with an operator's cab, Japanese Laid-Open Patent Application H6-108493 discloses a hydraulic power shovel which is provided with a cushioning support means consisting of a combination of coil springs compressedly interposed between a support frame and a cab body and oil dampers each having upper and lower ends thereof connected to the cab body and support frame.
The prior art hydraulic power shovel just mentioned contemplates to lower the intrinsic vibration frequency of the cab body by setting a small spring constant for the coil springs and at the same time to lower the vibration transmission rate from the support frame to the cab body by suppressing the resonance through vibration attenuating actions of the oil dampers.
On the other hand, in order to cope with problems which would result from the use of coil springs with a small spring constant, that is to say, in order to prevent the operator's cab from being tilted to an excessively large degree in lateral directions or in forward and rearward directions on the support frame due to reductions in rigidity of the support against vibrations in sideward or lateral directions of the operator's cab (rolling) and against vibrations in forward and rearward directions (pitching), a parallel link mechanism containing torsion bars, is provided between the cab body and support frame thereby to connect them more rigidly.
Nevertheless, in the case of this prior art working machine with an operator's cab, the parallel link mechanism contributes to increase the rigidity of the operator's cab against rolling and pitching but as a result vibrations of the working machine can be more easily transmitted to the operator's cab during excavating operations or the like.
In this instance, the vibration attenuating characteristics of the oil dampers, which are employed for the cushioning support means, are determined in such a way as to balance with a small spring constant which is set for the coil springs. Therefore, the oil dampers can efficiently damp the transmission of vibrations to the operator's cab through the cushioning support means.
However, since neither the parallel link mechanism nor the torsion bars have functions to attenuate vibrations, it is difficult to damp the transmission of vibrations to the operator's cab by means of the parallel mechanism and torsion bars. Consequently, there still remains a problem that the cab body can be put in resonance on the support frame to deteriorate the amenity of the working space within the operator's cab as well as the operationability of control levers which are provided in the operator's cab.